Rattle noise on 01 cavalier...any ideas?

i have a 2001 chevy cavalier. whenever i would start it, it would make a quiet squeal noise. Once it would run the noise would stop. Now, there is a loud rattle noise. It just sounds like metal rattling around. I'm taking it to the garage tomorrow but i was curious if anyone had any idea what it could be. it is coming from the right side under the hood. i stopped at a local garage and asked them if they could look quickly and be sure it was safe to drive. the guy said he couldn't say exactly what it was. he said it was coming from around my altenator, water pump, air conditioner thingy, and he said there were pulleys and belts. If it were something serious (like the altenator or water pump would I notice anything else?) There is no overheating or loss of power just simply a noise. Thanks.